The present invention relates to a polyphase motor such as a three-phase motor having a stator having six magnetic poles and four magnetic poles each composed of permanent magnet, and a four-phase motor having a stator having eight magnetic poles and four magnetic poles each composed of permanent magnet.
In a conventional three-phase motor (not shown) which is operated at an arbitrary speed and output, a rotor containing permanent magnets in the circumferential direction is fixed and disposed inside a stator having six magnetic poles, and by changing the exciting magnetic polarity of each magnetic pole of phases A, B, C of the stator, the rotor is rotated.
The three-phase motor having such a structure is controlled by a driving unit. That is, a first transistor series circuit, a second transistor series circuit and a third transistor series circuit are connected between +E terminal and grounding. A core wire of each phase is connected to a first joint of the transistor, a second joint point of the transistor, and a third joint of the transistor.
On the other hand, a micro computer is provided so as to input a speed instruction pulse for controlling the rotation speed to an AND gate and a rotation position of the three-phase motor is detected by a position detecting signal generating circuit provided with hall devices each time when the rotor is rotated by 30.degree.. The position detection signal is inputted to the AND gate circuit via a delay circuit. The delay circuit adjusts a timing for supplying the position detection signal to the AND gate circuit by controlling the delay of the position detection signal. An interval of the delay is controlled by the micro computer depending on load and acceleration of the motor.
The AND gate circuit obtains the logical product of a speed instruction pulse from the micro computer and a position detection signal from the delay circuit and its logical product output is outputted to a six-step ring counter. This ring counter changes an output to six output lines in succession depending on input of the logical product output from the AND gate circuit and repeats this procedure to supply the output to an invertor IC. The invertor IC fetches an output from the ring counter and controls switching of the respective transistors so that the three-phase motor is operated in step order for phase excitation or in inverse step order.
In this conventional three-phase motor, a voltage between respective lead wires Y or .DELTA. connected of three-phase windings for phases A, B, C is changed over between normal direction and inverse direction by switching operation of six transistors so as to drive the rotor 3. Therefore, switching frequency per a rotation is as large as six, and the fact that the switching frequency is large is an obstacle for high-speed driving of the motor. That is, to prevent a shortcircuit of a transistor bridge composed of transistors, a high side transistor needs to be actuated after a low side transistor is completely interrupted, and a waiting time is thus required. Therefore, an expensive power transistor having a high switching speed needs to be chosen as a transistor so as to achieve high-speed driving.
As described above, conventionally, to achieve high-speed driving, a number of expensive power transistors having a high switching speed have to be used, and therefore, there are disadvantages in that a space of a circuit portion and the production cost increase.